FIG. 1 shows a conventionally used spark plug 9 for an internal combustion engine. For example, the spark plug 9 is used as a means for igniting an air-fuel mixture introduced into a combustion chamber of an internal combustion engine such as of a passenger car.
The spark plug 9 includes a center electrode 94 and a ground electrode 95. The ground electrode 95 has an end fixed to a housing 92. The ground electrode 95 is bent to bring the other end to a position facing the center electrode 94. Thus, a spark discharge gap 911 is formed between the center electrode 94 and the ground electrode 95.
In the ground electrode 95, a projection portion 96 is arranged, being projected toward the spark discharge gap 911. The projection portion 96 has an opposing face 960 that faces the center electrode 94. As shown in FIG. 2 by (A) and (B), a discharge is caused in the spark discharge gap 911 and the air-fuel mixture is ignited by the discharge. A reference E in the figure indicates a discharge spark formed by the discharge, a reference F indicates a flow of the air-fuel mixture and a reference I indicates a flame (see Patent Document 1).
Patent Document 2 discloses a spark plug having a ground electrode in a shape that is engineered to minimize wear.